In digital data systems having phase modulated carriers, the carrier is modulated by adjusting its phase according to a selected number of degrees phase shifted. The amount of phase shift is quantified according to 360.degree. divided by the number of data that it is to be encoded. Typically, a four state per sample (di-bit) phase shift modulated data system will provide a carrier which is modulated at 90.degree. increments. The time interval over which the data modulates the carrier is a baud period typically comprising one or two cycles of the carrier frequency. A plot of the data signal over time provides a square wave-like signal having one of four amplitude values. A sequence of data signals to be transmitted at the baud rate reveals a step-like signal changing at periodic intervals. Since the rapid transition from one data value to another, through the phase shift modulated means, would consume an inordinate amount of spectrum space, and in view of the band pass filtering typically provided for modem reception, the transition points are broadened such that the sequence of step wise changes is reformed to a more gradual like signal trace forming the well known "eye" pattern. Previous data demodulators have provided for signal sampling only at the center of the open eye period, where the signal quality is felt to be most representative of the data which is used to modulate the carrier. However, receivers detecting the signal have difficulty in defining the center of the eye pattern and difficulty in providing a reliable phase detection at that point when the received signal is not well defined at the points where the signal is sampled.
In differentially encoded phase shift signals, the data is represented by the difference in relative signal phase between one baud period and the next. Typically, receivers of this nature compare a signal representative of one baud period with the signal of the subsequent baud period, wherein the detected phase data signal is provided by the comparison directly. However, when the signal becomes noisy such that the phase of one or both of the signal baud periods includes significant error signal modulation, error is added in both the present and subsequent baud intervals, reducing the noise sensitivity of the receiver.